Prosthetic limbs must be custom made because of the individual deviation in height and weight of each person and the individual idiosyncratic physiological condition of the person's residual limb including, but not limited to, the length of the residual limb, the possible weight fluctuations thereof and the atrophy of the limb that typically occurs after amputation. Moreover, the residual limb commonly changes shape due to the changes in swelling during the healing process. Because each prosthetic limb must be custom made to accommodate these individual conditions, such limbs cannot be mass produced, which considerably increases their costs.
In fitting a patient with a prosthesis following an amputation of a lower limb, the prosthetist must ensure the prosthesis swings substantially in the sagittal plane during walking by the patient. The body of the patient and his attitude or gait when walking typically require certain adjustments in the relative positioning of various components of the prosthetic device. These adjustments are frequently made in two orthogonal places--the anterior-posterior plane and in the lateral-medial plane. During the initial fitting, the prosthetist typically builds up an artificial limb utilizing adjustable elements in accord with the length and orientation of the patient's body. The final prosthetic device, however, is commonly permanently fixed at the various joints, thus precluding any further or later adjustment. An improper adjustment means that the patient wearing the leg prosthesis binds the leg unnaturally, which results in an unnatural movement pattern during walking. Nevertheless, even with the best initial fitting, the patient, while adapting to the artificial limb, may change his stance or gait to the extent that, for example, flexion of the knee joint no longer occurs in the sagittal plane. This is difficult to accommodate in a permanently bonded artificial limb, particularly when the prosthetic socket, normally molded to fit the patient's residual limb, is fixed to the remaining portion of the prosthesis.
Some adjustable fittings or connectors are available in the prior art to permit separation of the molded socket from the prosthesis to permit incremental rotation of the inferior portion of the prosthesis relative to the socket. Certain prior art devices also include later adjustment facilities to allow for the adjustment of the angular attitude and position relative to the load line of the pylon tube after the prosthesis has been in use for some time. (The load line is an imaginary line extending between the foot joint and the knee along which, ideally, the body weight acts.)
Various prosthetic joints or connectors for an endo-skeletal artificial leg are also well known. Such joints typically comprise an adjustable link designed to interconnect adjoining members of a prosthetic limb, such as a residual limb support, i.e., a prosthetic socket and a thigh member, a knee joint and a lower leg member, or at the ankle for connecting the lower end of a prosthesis to an artificial foot. The upper and/or lower portions of such an artificial joint is commonly provided with some means for adjustment.
One prior art system is the ball-and-socket type that permits appropriate flexion of the shin relative to the foot. Exemplary of such technology is Shorter et al., U.S. Pat. No. 4,463,459. Such ball-coupling arrangements, however, are generally of a heavy construction in order to achieve the required strength and stability while in use. The resulting heavy weight, however, is undesirable to the wearer as it causes undue energy expenditure and lack of control of the prosthetic device.
Moreover, various types of angle adjustment units are also known for adjusting the longitudinal axis of a prosthesis. Present modular prosthetic limb components commonly utilize frusto-pyramidal bosses and screws to affect angular adjustments in alignment and speed assembly procedures. Typically, a series of metal adapters and aluminum tubes are connected together to assembly the prosthetic structure. Such adapters employ only a relatively small surface area to interface parts. Consequently, heavy metals such as steel or titanium are typically used in such amounts that increase the weight of the device significantly. Thus, though such devices are manufactured for convenient later adjustability, they are not designed for minimum weight. Furthermore, these angular adjustment units are capable of transmitting only a relatively small momentum and are expensive to design and manufacture.
In any prosthetic device, it is desirable to decrease the weight of the elements in order to decrease the strain placed on the patient. Elimination of any unnecessary parts and the use of lighter materials to replace heavier components such as the connector joint or the pylon tube are particularly desirable objectives.
Some modular componentry, on the other hand, is relatively light in weight but lacks the desired adjustability. In the case of a modular below-knee (BK) prosthesis, the lightest a prosthetic device can be is about 3 pounds. For an above-knee (AK) prosthesis, the minimal weight is about 6 pounds to maintain full adjustability. This factor is significant because a prosthesis is considered "dead weight" (without sensation) to the patient. During the swing phase of gait, the prosthesis will tend to drop away from the patient's residual limb due to its weight. During the heel strike and stances phases, the prosthesis will tend to move upward until pressure equilibrium is attained. This results in considerable "pistoning" (up and down piston-like movements) of the prosthesis due to gravity, especially in the case of poor suspension of the prosthetic device from the residual limb. This pistoning action leads to lack of control and reduced proprioception. Reduction in weight can reduce or eliminate these problems by reducing the moment of inertia required to accelerate and decelerate the prosthesis.
Accordingly, there remains a need for a prosthetic device that is light in weight while sufficiently strong, that is economical to manufacture, and that readily allows for later adjustment of the device without damaging the physical integrity of the prosthetic device.